Share this story

Today is a good day: I learned something new. Many of you have probably heard of gravitational lensing of light (if not, don’t worry, I will inflict an explanation on you below). But it never occurred to me that gravitational waves are also subject to gravitational lensing. Not only that, but unexpectedly, the current generation of gravitational wave observatories might be able to observe gravitational lensing. If this turns out to be the case, all sorts of small, heavy dark objects might be revealed.

Specsavers don’t stock these lenses

One of Einstein’s predictions for general relativity was that light would be bent by a gravitational field. In effect, every planet, star, and galaxy bends light to a greater or lesser degree. We have used this effect to our advantage: distant galaxies provide lenses that allow us to see objects far beyond the galaxy itself. In some cases, distant objects appear as a ring around the lensing galaxy.

This effect is due to mass' distortion of space-time, and it will happen to any waves that pass nearby. Gravitational waves are just another form of wave, and, yes, they are also subject to gravitational fields from nearby masses. That means that a source of gravitational waves that passes through a lens should produce a similar ring of gravitational wave images as well.

However, our gravitational wave observatories don’t have the spatial resolution to see that ring. Effectively, we “see” a single blurry object if it is subject to gravitational lensing.

Seeing through the distortion

That view, though, is a distinctly optical one. Gravitational waves are not light waves, and if you consider where they come from, the situation gets far more interesting.

Currently, the only gravitational waves that we can detect come from mergers—a tame name for an Earth-shattering collision—between objects like black holes and neutron stars. These are the sumo wrestlers of the Universe: stars like the Sun are too small to even get an audition in the gravitational wave show.

The signal that we get, shown below, is called a chirp. Essentially, it is a vibration that starts in the base register and ends in the treble. The signal only exists for a few seconds before the sources merge. And it is this combination of the signal being short-lived and chirping that gives us the opportunity to detect the presence of a gravitational lens.

Enlarge/ The top two graphs show the predicted signal of gravitational waves, based on Kip Thorne's work, compared to what was seen at the two LIGO detectors.

Imagine our spiraling black holes as a single point in space. The gravitational waves head outward, traveling in apparently straight lines to us. But if there is a lens in between, some of the waves that would have missed Earth are bent back toward us. The bent waves have traveled farther, so they arrive a bit later than the waves that travel directly to us.

If the lens were really strong, we would observe two chirp signals: one from the waves that traveled straight to us and a second one a little bit later that came to us via the lens. This is the temporal version of the ring of images described above.

Even if the lens is weaker, however, the delay between the two chirps mixes up the frequencies. The two chirp signals overlap, but the delayed chirp has a lower frequency. At some points of time, the chirps will add up to produce a stronger signal, while at other points in time, the signal will be suppressed (I’ve reproduced an exaggerated version of this below).

This means that the expected chirp signal—one that smoothly increases in frequency and volume—is replaced by one that increases in frequency but oscillates in volume. This is something that we might be able to detect.

Enlarge/ Top signal is the chirp that arrives directly from the merger. The middle signal has been delayed by a gravitational lens. The bottom signal is the signal we would measure. An analysis of the measured signal may reveal the gravitational lens.

Chris Lee

Might?

That might is important. Gravitational wave detection relies on a number of different data processing techniques. Gravitational waves are buried in a lot of local Earth-shaking events (like bunny rabbits out for a run). To sort out the fakes from the real thing, physicists use multiple detectors to look for whole-Earth events. The rare events that are common to both detectors are then carefully compared to physical models to see if the results fit with our expectations for mergers.

The paper claims that, if the signal is subject to detectable lensing, it will no longer fit the predictions of current models. Does this mean such events will be discarded? I somehow doubt it but fear it could be the case.

You might be wondering why gravitational lensing for gravitational waves is interesting. To detect a gravitational lens, gravitational waves would have to pass close to very massive and compact objects.

How massive? Think primordial black holes with masses 10 to 100,000 times that of the Sun. No one really knows if or how many of these hugely massive black holes exist. Compact dark matter objects are another possibility. As for compact dark matter objects, that is another mystery. We are reasonably sure dark matter exists, but whether it clings to itself to form massive and dense objects is unknown.

In the end, that is why I like this research: it shows that gravitational wave detectors really are opening up a new view of our Universe. Their potential will continue to unfold before us in the coming four to five decades.

Share this story

Chris Lee
Chris writes for Ars Technica's science section. A physicist by day and science writer by night, he specializes in quantum physics and optics. He Lives and works in Eindhoven, the Netherlands. Emailchris.lee@arstechnica.com